The present invention relates generally to drilling bits for drilling underground formations, and more particularly, relates to drilling bits of the type commonly termed, "rolling cutter drill bits". Generally, these bits utilize a plurality of generally frusto-conical shaped cutter shells rotatably mounted on cantilevered bearing journals. Between the cutter shells and the bearing journals, there are usually a number of different bearing systems to take the loads and shocks encountered during the underground drilling. In one particular type of rolling cutter drill bit, a combination of ball bearings and so-called "friction" bearings are used to absorb these thrusts and loadings. The present invention is directed to the friction type bearing system, but is advantageous in all bearing systems utilized in rolling cutter drill bits.
Normally, the friction bearing type rolling cutter bit utilizes a combination of ball bearings and cylindrical bearing sleeves between the cutters and the bearing journals. The bearing sleeves are commonly referred to as journal bearings, or friction bearings. Usually, the bearing system in such a bit is lubricated by internal grease lubrication systems comprising channels and grease reservoirs. Also, these bits have precise sealing systems for maintaining these lubricants in the bearing system and for preventing entry of contaminants, such as well fluids and cuttings.
A second type of rolling cutter drill bit is also commonly used which utilizes both friction bearings and ball bearings, but which is not a sealed bit. Furthermore, this type of bit does not utilize internal lubricants, but instead, relies upon soft metal lubricants plated upon the bearing sleeves and/or the bearing surfaces. These soft metal lubricants usually comprise lead, copper, silver, indium, tin and various combinations of these metals. Often these unsealed rolling cutter bits are utilized in blast hole drilling for mining operations, and in water well drilling. The non-sealed, non-lubricated bits of this nature generally are operated with pressurized air to remove the rock cuttings from the bore hole as the bit rotates and drills through the rock. The air flow through the bit performs several different functions, including the aforementioned removal of cuttings, plus a cooling function and as a barrier against the entrance of contaminants and cuttings into the bearing structure. The primary disadvantage of the unsealed friction bearing type bit is that the lubricant coated on the bearing surfaces eventually becomes inadequate to lubricate the bearings, and rapid bearing failure soon occurs.
In some embodiments of the soft metal lubricant bearing system, the lubricating metal is actually inlaid into the bearing surface on the journal, or may be inlaid into recesses in the cutter bearing surface. As a result of the inlay, the lubricant is not available to the bearing system unless a wearing away of the base steel in the bearing surfaces also occurs. As a result, increased tolerances between the bearing surfaces begin to appear, and eventually the bearing fails and the bit is lost.
The present invention overcomes the disadvantages of the soft metal lubricant bearing systems in the unsealed bits utilizing air pressure rather than drilling fluids. This is accomplished through the present invention by the provision of lubricant rods, or sticks, formed in a solid or semi-solid state and located in channels in either the bearing journal, or the cutter, or both; with a biasing means behind the rods to continuously bias the rods against the bearing surfaces and thereupon provide a continuing supply of lubricant material to the bearing surfaces.